Information carriers having a plurality of information layers are widely used. For example, some DVDs (DVD stands for Digital Versatile Disc) comprise a first and a second information layers, which can be scanned by means of a radiation beam, in order to read from or record to one of these information layers. The expression “scanning” means either reading or writing data from or to an information layer.
An optical scanning device for scanning such a dual-layers DVD comprises a radiation source for generating a diverging radiation beam, a collimator for converting this diverging beam into a parallel beam and an objective lens for focussing said parallel beam on one of the two information layers. The optical scanning device comprises an actuator for moving the objective lens axially in order to focus the parallel beam on the desired information layer.
When the objective lens is moved in order to jump from the first information layer to the second information layer, a certain amount of spherical aberration occurs. However, in the case of a DVD, this amount of spherical aberration is relatively low, because the numerical aperture of the radiation beam is relatively low and the wavelength relatively high. The amount of spherical aberration is indeed within the tolerances of the optical scanning device, so that no spherical aberration correction is needed. Hence, when jumping from one information layer to another, only a refocusing action is required.
Now, in order to increase the capacity of an information layer, a next generation optical disc system has been designed, which uses a higher numerical aperture and a lower wavelength of the radiation beam. For example, a BD player and/or recorder uses an objective lens with a numerical aperture of 0.85 and a radiation beam with a wavelength of 405 nanometers. As a consequence, the amount of spherical aberration that occurs when jumping from one layer to another is higher than for a DVD player. Actually, the amount of spherical aberration is proportional to the fourth power of the numerical aperture, which means that a slight increase in the numerical aperture leads to a large increase in the amount of spherical aberration. Typically, for a dual-layer BD, the amount of spherical aberration that occurs when jumping from one layer to another is about 250 ml rms, which is out of the tolerances of the optical scanning device. As a consequence, it is necessary to correct for the spherical aberration when jumping from one layer to another.
A way of correcting for the spherical aberration when jumping from the first information layer to the second information layer consists in designing the objective lens so that no spherical aberration occurs when a parallel entrance beam is focussed on the first information layer, i.e. when an infinite conjugate is used. When jumping to the second information layer, the vergence of the entrance beam is changed, i.e. a finite conjugate is used. For example, a converging entrance beam is focussed on said second information layer. The objective lens is designed to significantly comply with the sine condition in order to have a substantial field of view. A lens designed to comply with the sine condition is known to give rise to spherical aberration when the conjugate distance compared to the design situation is changed. This is explained, for example, in “Principles of Optics”, by M. Born and E. Wolf, Pergamon Press, Oxford, 1993, p166-169. As a consequence, by changing the conjugate distance, i.e. the vergence of the entrance beam, it is possible to generate spherical aberration, which compensates the spherical aberration that occurs when jumping from one layer to another.
However, due to eccentricity of the spirals of the information carrier, the objective lens has to be moved away from its central position during scanning, that is to say a decentring of the objective lens occurs during scanning. Now, when a finite conjugate is used for scanning the second information layer, the objective lens is sensitive for decentring. The higher the vergence of the entrance beam, the more sensitive for decentring the objective lens. As a consequence, the possible decentring of the objective lens of an optical scanning device as described above is limited, and such an optical scanning device cannot scan an information carrier presenting a relatively high eccentricity of the spirals.